MEMS-based rotation sensor for seismic applications and sensor units having same

ABSTRACT

The present disclosure is directed to a MEMS-based rotation sensor for use in seismic data acquisition and sensor units having same. The MEMS-based rotation sensor includes a substrate, an anchor disposed on the substrate and a proof mass coupled to the anchor via a plurality of flexural springs. The proof mass has a first electrode coupled to and extending therefrom. A second electrode is fixed to the substrate, and one of the first and second electrodes is configured to receive an actuation signal, and another of the first and second electrodes is configured to generate an electrical signal having an amplitude corresponding with a degree of angular movement of the first electrode relative to the second electrode. The MEMS-based rotation sensor further includes closed loop circuitry configured to receive the electrical signal and provide the actuation signal. Related methods for using the MEMS-based rotation sensor in seismic data acquisition are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/739,602 filed Dec. 19, 2012, which isincorporated herein by reference in its entirety.

BACKGROUND

Seismic exploration involves surveying subterranean geologicalformations for hydrocarbon deposits. A seismic survey typically involvesdeploying seismic source(s) and seismic sensors at predeterminedlocations. The sources generate seismic waves, which propagate into thegeological formations creating pressure changes and vibrations alongtheir way. Changes in elastic properties of the geological formationscatter the seismic waves, changing their direction of propagation andother properties. Part of the energy emitted by the sources reaches theseismic sensors. Some seismic sensors are sensitive to pressure changes(hydrophones), others to particle motion (e.g., geophones), andindustrial surveys may deploy only one type of sensors or both. Inresponse to the detected seismic events, the sensors generate electricalsignals to produce seismic data. Analysis of the seismic data can thenindicate the presence or absence of probable locations of hydrocarbondeposits.

Historically, seismic data acquisition along a surface has beenaccomplished by placing seismic sources and sensors along a straightline. In such a configuration, it is assumed that the reflection pointsin the ground are located in a two-dimensional plane delimited by thetransverse line and the vertical axis. This is often referred to as atwo-dimensional seismic survey. However, three-dimensional seismicsurveys are often preferred in order to obtain better signal quality andto improve the space and the time resolution. One of the drawbacks ofthree-dimensional surveys is the requirement of a large amount ofsensors, which necessitates a large deployment crew. This results inincreased costs and decreased efficiency. Accordingly, improved seismicsensors allowing for sparse sampling and thus less deployment of sensorswithout compromising data quality are desired.

BRIEF SUMMARY

The present disclosure is directed to a MEMS-based rotation sensor foruse in seismic data acquisition and sensor units having same. A sensorunit for land-based seismic data acquisition includes a particle motionsensor for measuring a vertical wavefield in which the verticalwavefield has a horizontal gradient. The sensor unit further includes afirst MEMS-based rotational accelerometer for measuring an x-componentof the horizontal gradient and a second MEMS-based rotationalaccelerometer positioned orthogonally to the first MEMS-based rotationalaccelerometer. The second MEMS-based rotational accelerometer measures ay-component of the horizontal gradient. At least one of the first andsecond MEMS-based rotational accelerometers includes a substrate, ananchor disposed on the substrate, and a proof mass coupled to the anchorvia a plurality of flexural springs. The proof mass has a firstelectrode coupled to and extending therefrom. A second electrode isfixed to the substrate, and one of the first and second electrodes isconfigured to receive an actuation signal, and another of the first andsecond electrodes is configured to generate an electrical signal havingan amplitude corresponding with a degree of angular movement of thefirst electrode relative to the second electrode. The MEMS-basedrotation sensor further includes closed loop circuitry configured toreceive the electrical signal and provide the actuation signal.

A seismic data acquisition system is described having one or moresources for generating seismic waves and one or more sensor units forrecording seismic waves generated by the sources. The one or more sensorunits include a first seismic sensor for measuring a vertical wavefieldof the seismic waves, and a second seismic sensor for measuring agradient of the vertical wavefield.

A method for performing seismic data acquisition is also described. Themethod includes generating seismic waves using one or more sources, andrecording seismic waves generated by the sources using one or moresensor units. The one or more sensor units include a first seismicsensor for measuring a vertical wavefield of the seismic waves and asecond seismic sensor for measuring a gradient of the verticalwavefield.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings.

FIG. 1 illustrates a schematic view of a MEMS-based rotation sensoraccording to the present disclosure;

FIG. 2 illustrates rotation of the MEMS-based rotation sensor of FIG. 1;

FIG. 3 illustrates a schematic view of a MEMS-based rotation sensoraccording to another embodiment of the present disclosure;

FIG. 4 illustrates a flowchart of a control loop of a MEMS-basedrotation sensor according to the present disclosure;

FIG. 5 illustrates a schematic view of a MEMS-based rotation sensoraccording to the present disclosure;

FIG. 6 illustrates a schematic view of a MEMS-based rotation sensoraccording to the present disclosure;

FIG. 7 illustrates a schematic view of a Nyquist sampling;

FIG. 8 illustrates a schematic view of a Papoulis sampling;

FIG. 9 illustrates a schematic view of a land-based seismic dataacquisition system incorporating a MEMS-based rotation sensor accordingto the present disclosure;

FIG. 10 illustrates a schematic view of a sensor unit incorporating aMEMS-based rotation sensor according to the present disclosure; and

FIG. 11 illustrates a marine-based seismic data acquisition systemincorporating a MEMS-based rotation sensor according to the presentdisclosure.

DETAILED DESCRIPTION

Various embodiments of a MEMS-based rotation sensor and methods of usingsuch a MEMS-based rotation sensor according to the present disclosureare described. It is to be understood, however, that the followingexplanation is merely exemplary in describing the devices and methods ofthe present disclosure. Accordingly, several modifications, changes andsubstitutions are contemplated.

FIG. 1 schematically illustrates mechanical structure of a capacitivemicroelectromechanical system (MEMS)-based rotation sensor 10 accordingto one embodiment of the present disclosure. The MEMS-based design ofthe sensor 10 is advantageous due to its size, low power dissipation andlow cost. The MEMS-based rotation sensor 10 includes a central anchor 12coupled to a seismic mass 14 via a plurality of flexure springs 16. Insome embodiments, the central anchor 12 has a radius of approximately300 μm, while the seismic mass 14 has a width of approximately 300 μm.Of course, other embodiments are contemplated in which the centralanchor 12 and seismic mass 14 have other dimensions. The anchor 12 iscoupled to the outside environment and thus experiences angularaccelerations, which causes the seismic mass 14 to rotate due toinertial effects. The flexure springs 16 are able to bend, thuspermitting rotation of the seismic mass 14 about anchor 12.

Referring to FIG. 2, the MEMS-based rotation sensor 10 includes a pairof beam-shaped electrodes 18, 20, which are disposed at opposing sidesof the sensor. In some embodiments, such electrodes 18, 20 have a lengthof 550 μm and a width of 5 μm. Of course, other embodiments arecontemplated in which the beam-shaped electrodes 18, 20 have otherdimensions. The pair of electrodes 18, 20 cooperates to detectdifferential capacitance resulting from angular displacement of theMEMS-based rotation sensor. In the example of FIG. 2, electrode 18 isdisplaced from its origin position 22 as a result of the angulardisplacement of the seismic mass 14. Electrode 20 is also displaced inan opposite direction (relative to displacement of electrode 18) fromits origin position 24 as a result of the displacement of the seismicmass 14. As a result, electrode 18 increases its capacitance of aquantity ε, while electrode 20 decreases its capacitance of the samequantity ε. The capacitance difference of the two electrodes 18, 20gives the capacitance variation quantity ε, which is related to therotation of the seismic mass 14. Angular displacements of the seismicmass 14 are thus converted into electrical signals using such adifferential capacitor detector. It will be appreciated that therespective change in capacitance at the electrodes 18 and 20 may also bedifferent quantities, may vary by a linear function, and may also varyby a non-linear function.

Referring to FIG. 3, a MEMS-based rotation sensor 30 according toanother embodiment of the present disclosure is illustrated to includesliding comb electrodes 32, 34 which are used for detection ofdifferential capacitance resulting from rotation of a seismic mass 36.In some embodiments, the sliding comb electrodes 32, 34 have a fingerlength of 20 μm, a finger width of 3 μm, a comb length of 420 μm, and acomb width of 50 μm. Of course, other dimensions are contemplated. TheMEMS-based rotation sensor 30 further includes a central anchor 38 andflexural springs 39, which couple the seismic mass 36 to the centralanchor. The comb electrodes 32, 34 generate electromechanical efforts inorder to balance seismic angular displacements at the same time asdifferential capacitance detection. Capacitance variations of theMEMS-based rotation sensor 30 generate a current, which passes through acharge amplifier to obtain an output voltage. At this point, open-loopreadout is achieved whereby the output voltage is proportional to theinput angular accelerations measured by the MEMS-based rotation sensor30.

The MEMS-based rotation sensors described herein may be used withfeedback control loop architecture that linearizes the force functionwith respect to a control voltage to thereby increase dynamicperformances. This can be accomplished with analog control, digitalcontrol, or a combination thereof. An actuation signal such as a squarewave, triangle wave, sinusoid, or other waveform may be applied to oneor more electrodes (e.g., electrodes 32, 34). In response to theactuation signal, one or more of the electrodes (e.g., electrodes 32,34) may generate an electrical signal having a property, such asamplitude, corresponding to angular acceleration. The electrical signalcorresponding to angular acceleration may also be generated without anactuation signal. With reference to FIG. 4, feedback control looparchitecture 40 may include a sigma-delta modulator 42, which receivesthe electrical signal corresponding to angular acceleration and may beused to convert the measurement signal into a bit-stream voltage output.The output voltage may be passed through an integrator 44, a chargeamplifier 46, and a proportional-integral-derivative (PID) controller 48before applying a coarse quantization process 50. The analog signal maybe converted into a bit-stream sequence. The bit-stream may then be sentto a force feedback generator 52, which may provide the actuation signalto electrodes of the MEMS-based rotation sensor (e.g., electrode 32) tocreate electrostatic forces proportional to the bit-stream average. Thismay physically damp the oscillations of the seismic mass 14 (e.g., proofmass) by applying the electrostatic force thereto. This may beaccomplished by keeping the voltage levels constant and modulating theaverage force by pulse-density control or other control algorithms. Sucha process leverages principles underlying sigma-delta modulation, suchas oversampling of an analog signal, bit-stream conversion of an analogsignal (where a bit-stream average is a measure of the input signal) andoversampling to cause the quantization noise to spread over a widebandwidth. It will be appreciated that the electrodes may be arrangedsuch that a first set of the electrodes receive the actuation signal anda second set of the electrodes generate the electrical signalcorresponding to angular acceleration. The first and second sets may ormay not overlap. Some, or all, of the electrodes may receive theactivation signal and also generate the electrical signal correspondingto angular acceleration.

The present disclosure contemplates several variations of how electrodesare distributed around the seismic mass. For example, referring to FIG.5, a MEMS-based rotation sensor 60 includes a seismic mass 62 disposedabout a central anchor 64. The seismic mass 62 is coupled to the anchor64 via a plurality of flexural springs 66. In one embodiment, theMEMS-based rotation sensor 60 includes four flexural springs 66. Theseismic mass 62 includes a plurality of electrodes 68 (e.g., beam-shapedelectrodes) extending from and adapted to move with the seismic mass.The MEMS-based rotation sensor 60 is disposed on a substrate 70, whichincludes a pair of fixed electrodes 72, 74 disposed at opposing sides ofthe seismic mass 62. A trench 76 separates the areas of the substrate 70having the electrodes 72, 74.

Referring to FIG. 6, in another embodiment, a MEMS-based rotation sensor80 includes a seismic mass 82 coupled to a central anchor 84 via aplurality of flexural springs 86. The seismic mass 82 includes aplurality of electrodes 88 (e.g., in a beam configuration) extendingfrom and adapted to move with the seismic mass. In this embodiment,electrodes 90, 92 are split and distributed around the seismic mass 82.The MEMS-based rotation sensor 80 is disposed on a substrate 92 andincludes a pair of trenches 94, 96 to separate the areas of thesubstrate having the electrode sets 88, 90.

It is to be appreciated that several variations of the MEMS-basedrotation sensor described herein are contemplated. For example, gapclosing combs, sliding combs and sliding masses may be employed. Also,different control mechanism may be used, including direct capacitancemeasurements, differential capacitance measurements in an open loop, anddifferential capacitance measurements in a closed loop with forcefeedback. Furthermore, it is contemplated that the MEMS-based rotationsensor described herein may be used with a variety of other seismicsensors. For example, when used with a translational accelerometer, theMEMS-based rotation sensor would measure the gradient of any signalsmeasured by the translational accelerometer. Indeed, in suchembodiments, the translational accelerometer may be a MEMS-basedtranslational accelerometer that utilizes the same feedback control looparchitecture 40 (FIG. 4) as the MEMS-based rotation sensor.

The MEMS-based rotation sensors described herein may be used in theseismic data acquisition context to reduce the number of sensor nodesand/or increase the spacing among such nodes, thus resulting in largerdeployable arrays and/or lower operating costs. More particularly, theMEMS-based rotation sensors described herein may be used to measure thegradient of any signal acquired by translational accelerometers deployedin a seismic survey. In land seismic surveys, for example, surface waves(e.g., ground roll waves) have an apparent wavelength close to theirtrue wavelength since they are propagating with a large emergent angle.Surface waves typical of land seismic operation noises have higheramplitudes due to their stronger energy and their small apparentwavelength at the free surface.

The MEMS-based rotation sensors described herein are well-suited tomeasure the spatial gradients of noise components at the free surface.As a result of such gradient measurements, it is possible to interpolatenoise components between sensor nodes, thus allowing for sparser spatialsampling. For example, rather than employing a standard Nyquist sampling98 requiring two measurements for the shortest wavelength of a signal(FIG. 7), the MEMS-based rotation sensors according to the presentdisclosure permit Papoulis sampling 99, which only requires co-locatedmeasurements of the vertical wavefield and its gradient at each cyclefor the shortest wavelength (FIG. 8). Accordingly, deploying MEMS-basedrotation sensors according to the present disclosure requires fewersensors for seismic data acquisition without compromising data quality.Further, deployment of MEMS-based rotation sensors described hereinallows for local noise attenuation without using data from othersensors. Accordingly, local noise attenuation can be accomplishedindependent of sensor spacing. In one embodiment, rotation measurementdata may be used as a noise model for adaptive subtraction of groundroll noise.

The MEMS-based rotation sensors described herein (e.g., MEMS-basedrotation sensors 10, 30, 60, 80) may be used in a variety of seismicdata acquisition systems. For example, with reference to FIG. 9, anarrangement of sensor assemblies 100 that are used for land-basedseismic surveying are deployed on a ground surface 102. The groundsurface 102 overlies a geological formation 104 of interest, such as ahydrocarbon reservoir. One or more seismic sources 106, which can bevibrators, air guns, or explosive devices, are deployed in a surveyfield in which the sensor assemblies 100 are located.

Activation of the seismic sources 106 causes seismic waves to bepropagated into the geological formation 104. The seismic waves are thenreflected from subterranean structure 108 (including geologicalformation 104) and are propagated upwardly towards the sensor assemblies100. Sensors within the sensor assemblies measure the seismic wavesreflected from the subterranean structure 108. For example, referring toFIG. 10, an exemplary sensory assembly 100 is illustrated to include apair of MEMS-based rotation sensors (e.g., rotation sensor 60) accordingto the present disclosure as well as a MEMS-based translationalaccelerometer 110. It is to be appreciated that the MEMS-based rotationsensors 60 shown in FIG. 10 may be replaced with any MEMS-based rotationsensor disclosed herein. In the embodiment of FIG. 10, the sensorassembly 100 includes the MEMS-based rotation sensors 60 positionedorthogonally to one another such that one of the MEMS-based rotationsensors measures the x-component of the horizontal gradient of thevertical wavefield and the other of the MEMS-based rotation sensormeasures the y-component of the horizontal gradient of the verticalwavefield. The sensor assembly 100 may further include one or morespikes 112, or coupling elements, to improve coupling of the sensorassembly to the ground surface. According to another embodiment, thesensor assembly 100 may be entirely or partially inserted into theground, proximate the surface, so as to be coupled to the ground andsensitive to ground roll. According to an embodiment, the sensorassembly 100 can extend across the ground surface.

Referring again to FIG. 9, in one embodiment, the sensor assemblies 100are interconnected by an electrical cable 114 (or other type ofcommunication medium) to a signal processing unit 150. Alternatively,instead of connecting the sensor assemblies 100 by the electrical cable114, the sensor assemblies can communicate wirelessly with thecontroller signal processing unit (for cable-free sensor assemblies). Insome implementations, intermediate routers or concentrators may beprovided at intermediate points of the network of sensor assemblies 100to enable communication between the sensor assemblies and the signalprocessing unit 150.

The signal processing unit 150 shown in FIG. 9 further includesprocessing software 152 that is executable on a processor 154. Theprocessor 154 is connected to storage media 156 (e.g., one or moredisk-based storage devices and/or one or more memory devices). Thestorage media 156 is used to store sensor data 158, which includesoutput data produced by each of the sensor assemblies 100. In operation,the software 152 in the signal processing unit 150 is executable toprocess the sensor data 158 to produce an output to characterize thegeological formation 104. It should be appreciated that the items in theprocessing unit 150 can be incorporated into the sensor units 100. Thiscould be used in a “blind node” arrangement, where individual nodes areself contained with respect to collected data and/or power and/orcommunication.

The MEMS-based rotation sensors 10, 30, 60, 80 described herein may bealso be used in marine seismic data acquisition systems. For example,FIG. 11 depicts an embodiment of a marine-based seismic data acquisitionsystem 200 in accordance with some embodiments of the presentdisclosure. In the system 200, a survey vessel 202 tows one or moreseismic streamers 204 (one exemplary streamer 204 being depicted in FIG.11) behind the vessel. It is noted that the streamers 204 may bearranged in a spread in which multiple streamers are towed inapproximately the same plane at the same depth. As another example, thestreamers 204 may be towed at multiple depths, such as in an over/underspread, for example.

The seismic streamers 204 may be several thousand meters long and maycontain various support cables (not shown), as well as wiring and/orcircuitry (not shown) that may be used to support communication alongthe streamers. In general, each streamer 204 includes a primary cableinto which is mounted seismic sensors that record seismic signals. Inaccordance with embodiments of the present disclosure, the streamers 204contain seismic sensor units 206, which may include a hydrophone,particle motion sensors and the MEMS-based rotation sensor 10, 30, 60,80 described herein. Thus, each sensor unit 206 is capable of detectinga pressure wavefield and at least one component of a particle motionthat is associated with acoustic signals that are proximate to thesensor. Examples of particle motions include one or more components of aparticle displacement, one or more components (inline (x), crossline (y)and vertical (z) components) of a particle velocity and one or morecomponents of a particle acceleration. Each sensor unit 206 is furthercapable of detecting angular accelerations of vibration noise.

Depending on the particular embodiment of the present disclosure, thesensor units 206 may include one or more hydrophones, geophones,particle displacement sensors, particle velocity sensors,accelerometers, pressure gradient sensors, rotation sensors orcombinations thereof. For example, the sensor units 206 may include acapacitive microelectromechanical system (MEMS)-based sensor that issensitive to translational accelerations and the MEMS-based rotationsensor 10, 30, 60, 80 that is sensitive to angular accelerations.

The marine seismic data acquisition system 200 further includes seismicsources 208 (two exemplary seismic sources 208 being depicted in FIG.11), such as air guns and the like. In some embodiments of the presentdisclosure, the seismic sources 208 may be coupled to, or towed by, thesurvey vessel 202. Alternatively, in other embodiments, the seismicsources 208 may operate independently of the survey vessel 202, in thatthe sources may be coupled to other vessels or buoys.

As the seismic streamers 204 are towed behind the survey vessel 202,acoustic signals 210, often referred to as “shots,” are produced by theseismic sources 208 and are directed down through a water column 212into strata 214 and 216 beneath a water bottom surface 218. The acousticsignals 210 are reflected from the various subterranean geologicalformations, such as an exemplary formation 220 that is depicted in FIG.11.

The incident acoustic signals 210 that are created by the sources 208produce corresponding reflected acoustic signals, or pressure waves 222,which are sensed by the seismic sensors of the streamer(s) 204. It isnoted that the pressure waves that are received and sensed by theseismic sensors include “up going” pressure waves that propagate to thesensors without reflection, as well as “down going” pressure waves thatare produced by reflections of the pressure waves 222 from an air-waterboundary, or free surface 224.

The seismic sensors of the streamer(s) 204 generate signals (digitalsignals, for example), called “traces,” which indicate the acquiredmeasurements of the pressure wavefield and particle motion. The tracesare recorded and may be at least partially processed by a signalprocessing unit 226 (e.g., a unit the same or similar to the signalprocessing unit 150 of FIG. 9) that is deployed on the survey vessel202, in accordance with some embodiments of the present disclosure. Forexample, a particular multi-component sensor may provide a trace, whichcorresponds to a measure of a pressure wavefield by its hydrophone; andthe sensor may provide one or more traces that correspond to one or morecomponents of particle motion. The sensor unit may further sense angularaccelerations via the MEMS-based rotation sensor described herein. Suchmeasurements facilitate the removal of transverse vibration noise.

The goal of the seismic acquisition is to build up an image of a surveyarea for purposes of identifying subterranean geological formations,such as the exemplary geological formation 220. Subsequent analysis ofthe representation may reveal probable locations of hydrocarbon depositsin subterranean geological formations. In some embodiments, portions ofthe analysis of the representation may be performed on the seismicsurvey vessel 202, such as by the signal processing unit 226. Inaccordance with other embodiments, the representation may be processedby a seismic data processing system located remotely of the vessel 202.Thus, many variations are possible and are within the scope of theappended claims.

While various embodiments of a MEMS-based rotation sensor and relatedmethods of using MEMS-based rotation sensors have been described above,it should be understood that they have been presented by way of exampleonly, and not limitation. For example, while the MEMS-based rotationsensor 10 is described for use in seismic data acquisition systems, itis to be appreciated that the sensor may be used in other dataacquisition systems outside of the field of seismic data acquisition.Thus, the breadth and scope of the present disclosure should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents. Moreover, the above advantages and features are provided indescribed embodiments, but shall not limit the application of the claimsto processes and structures accomplishing any or all of the aboveadvantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, the “Background” is not to beconstrued as an admission that technology is prior art to anyinvention(s) in this disclosure. Neither is the “Brief Summary” to beconsidered as a characterization of the invention(s) set forth in theclaims found herein. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty claimed in this disclosure. Multipleinventions may be set forth according to the limitations of the multipleclaims associated with this disclosure, and the claims accordinglydefine the invention(s), and their equivalents, that are protectedthereby. In all instances, the scope of the claims shall be consideredon their own merits in light of the specification, but should not beconstrained by the headings set forth herein.

What is claimed is:
 1. A sensor unit for land-based seismic dataacquisition, comprising: a particle motion sensor for measuring avertical wavefield, the vertical wavefield having a horizontal gradient;a first MEMS-based rotational accelerometer for measuring an x-componentof the horizontal gradient; a second MEMS-based rotational accelerometerpositioned orthogonally to the first MEMS-based rotationalaccelerometer, the second MEMS-based rotational accelerometer formeasuring a y-component of the horizontal gradient; wherein at least oneof the first and second MEMS-based rotational accelerometers comprises:a substrate; an anchor disposed on the substrate; a proof mass coupledto the anchor via a plurality of flexural springs, the proof mass havinga first electrode coupled to and extending from the proof mass; a secondelectrode fixed to the substrate, one of the first and second electrodesbeing configured to receive an actuation signal, and another of thefirst and second electrodes being configured to generate an electricalsignal having an amplitude corresponding with a degree of angularmovement of the first electrode relative to the second electrode; andclosed loop circuitry configured to receive the electrical signal andprovide the actuation signal.
 2. A sensor unit according to claim 1,wherein the particle motion sensor and the first and second MEMS-basedrotational accelerometers are disposed in a housing, the housing havingone or more coupling elements extending downwardly therefrom, thecoupling elements providing coupling of the sensor assembly to a groundsurface.
 3. A sensor unit according to claim 1, wherein the particlemotion sensor is a one-component particle motion sensor.
 4. A sensorunit according to claim 1, wherein the particle motion sensor is athree-component particle motion sensor.
 5. A sensor unit according toclaim 1, wherein the first electrode is a beam-shaped electrode.
 6. Asensor unit according to claim 1, wherein the first and secondelectrodes are comb-shaped electrodes.
 7. A sensor unit according toclaim 1, wherein the closed loop circuitry includes a sigma-deltamodulator and the electrical signal passes through the sigma-deltamodulator.
 8. A sensor unit according to claim 1, wherein the electricalsignal is an analog signal and the closed loop circuitry converts theanalog signal into a bit stream sequence.
 9. A sensor unit according toclaim 1, wherein the closed loop circuitry further comprises a forcefeedback generator for receiving the bit stream sequence and convertingthe bit stream sequence into the actuation signal.
 10. A sensor unitaccording to claim 1, wherein the particle motion sensor includes athird MEMS-based rotational accelerometer for measuring a gradient ofthe vertical wavefield, the third MEMS-based rotational accelerometerbeing positioned orthogonally to the second MEMS-based rotationalaccelerometer.
 11. A seismic data acquisition system, comprising: one ormore sources for generating seismic waves; and one or more of the sensorunits of claim
 1. 12. A method for performing seismic data acquisition,comprising: generating seismic waves using one or more sources;recording seismic waves generated by the sources using one or more ofthe sensor units of claim
 1. 13. A sensor unit according to claim 1,wherein the anchor is disposed centrally of the proof mass.
 14. A sensorunit according to claim 1, wherein the closed loop circuitry isconfigured to damp oscillations of the proof mass.